It has long been desired to operate internal combustion engines, in particular engines with multiple combustion cylinders, in a manner which results in optimal performance with minimal emissions. This goal, however, is difficult to achieve. One contributing factor is the lack of uniform combustion characteristics from cylinder to cylinder. Variables such as air mass, temperature and pressure imbalances contribute to this non-uniformity. The problem may be made worse by the use of intake valve actuation techniques, such as Miller cycle, which may induce a backflow of air from cylinders into input ports. The result may include differences in peak cylinder pressure, rise rates and emissions from each cylinder, caused by non-uniform combustion phasing among the cylinders.
Efforts to improve performance and emissions characteristics of internal combustion engines have shown promise with the development of homogeneous charge compression ignition (HCCI) techniques. However, the variations in combustion phasing between cylinders, caused by the above variables, poses a significant obstacle to making HCCI engines a viable low emissions technology.
Attempts have-been made to “tune” cylinder to cylinder variations in an engine. For example, in U.S. Pat. No. 6,276,334, Flynn et al. describe a method which adjusts the timing of exhaust valves to minimize cylinder to cylinder variations. The method adjusts the start of combustion timing for each cylinder, thus enabling the cylinders to combust at substantially the same time relative to each other. However, by varying the timing of the exhaust valves, the temperature within each cylinder increases, which in turn may lead to higher emissions being generated. Furthermore, when Miller cycle is being used, varying the timing of the exhaust valves does not directly address the non-uniformities being created by the variable intake valve timing process.
The present invention is directed to overcoming one or more of the problems as set forth above.